AGW Observer

Observations of anthropogenic global warming

New research from last week 37/2011

Posted by Ari Jokimäki on September 19, 2011

Here is the new research published last week. I’m not including everything that was published but just some papers that got my attention. Those who follow my Facebook page (and/or Twitter) have already seen most of these, as I post these there as soon as they are published. Here, I’ll just put them out in one batch. Sometimes I might also point out to some other news as well, but the new research will be the focus here. Here’s the archive for the news of previous weeks. By the way, if this sort of thing interests you, be sure to check out A Few Things Illconsidered, they have a weekly posting containing lots of links to new research and other climate related news.

It’s Trenberth et al. turn to pick on Spencer & Braswell

Issues in Establishing Climate Sensitivity in Recent Studies – Trenberth et al. (2011) “Numerous attempts have been made to constrain climate sensitivity with observations [1-10] (with [6] as LC09, [8] as SB11). While all of these attempts contain various caveats and sources of uncertainty, some efforts have been shown to contain major errors and are demonstrably incorrect. For example, multiple studies [11-13] separately addressed weaknesses in LC09 [6]. The work of Trenberth et al. [13], for instance, demonstrated a basic lack of robustness in the LC09 method that fundamentally undermined their results. Minor changes in that study’s subjective assumptions yielded major changes in its main conclusions. Moreover, Trenberth et al. [13] criticized the interpretation of El Niño-Southern Oscillation (ENSO) as an analogue for exploring the forced response of the climate system. In addition, as many cloud variations on monthly time scales result from internal atmospheric variability, such as the Madden-Julian Oscillation, cloud variability is not a deterministic response to surface temperatures. Nevertheless, many of the problems in LC09 [6] have been perpetuated, and Dessler [10] has pointed out similar issues with two more recent such attempts [7,8]. Here we briefly summarize more generally some of the pitfalls and issues involved in developing observational constraints on climate feedbacks.” Kevin E. Trenberth, John T. Fasullo and John P. Abraham, Remote Sens. 2011, 3(9), 2051-2056; doi:10.3390/rs3092051. [Full text]

Determining the contributors to Earth’s sea level and energy budget

Revisiting the Earth’s sea-level and energy budgets from 1961 to 2008 – Church et al. (2011) “We review the sea-level and energy budgets together from 1961, using recent and updated estimates of all terms. From 1972 to 2008, the observed sea-level rise (1.8 ± 0.2 mm yr−1 from tide gauges alone and 2.1 ± 0.2 mm yr−1 from a combination of tide gauges and altimeter observations) agrees well with the sum of contributions (1.8 ± 0.4 mm yr−1) in magnitude and with both having similar increases in the rate of rise during the period. The largest contributions come from ocean thermal expansion (0.8 mm yr−1) and the melting of glaciers and ice caps (0.7 mm yr−1), with Greenland and Antarctica contributing about 0.4 mm yr−1. The cryospheric contributions increase through the period (particularly in the 1990s) but the thermosteric contribution increases less rapidly. We include an improved estimate of aquifer depletion (0.3 mm yr−1), partially offsetting the retention of water in dams and giving a total terrestrial storage contribution of −0.1 mm yr−1. Ocean warming (90% of the total of the Earth’s energy increase) continues through to the end of the record, in agreement with continued greenhouse gas forcing. The aerosol forcing, inferred as a residual in the atmospheric energy balance, is estimated as −0.8 ± 0.4 W m−2 for the 1980s and early 1990s. It increases in the late 1990s, as is required for consistency with little surface warming over the last decade. This increase is likely at least partially related to substantial increases in aerosol emissions from developing nations and moderate volcanic activity.” Church, J. A., N. J. White, L. F. Konikow, C. M. Domingues, J. G. Cogley, E. Rignot, J. M. Gregory, M. R. van den Broeke, A. J. Monaghan, and I. Velicogna (2011), Geophys. Res. Lett., 38, L18601, doi:10.1029/2011GL048794.

La Niña more frequent than El Niño during Medieval Climate Anomaly

Assessing El Niño Southern Oscillation variability during the past millennium – Khider et al. (2011) “We present a reconstruction of El Niño Southern Oscillation (ENSO) variability spanning the Medieval Climate Anomaly (MCA, A.D. 800–1300) and the Little Ice Age (LIA, A.D. 1500–1850). Changes in ENSO are estimated by comparing the spread and symmetry of δ18O values of individual specimens of the thermocline-dwelling planktonic foraminifer Pulleniatina obliquiloculata extracted from discrete time horizons of a sediment core collected in the Sulawesi Sea, at the edge of the western tropical Pacific warm pool. The spread of individual δ18O values is interpreted to be a measure of the strength of both phases of ENSO while the symmetry of the δ18O distributions is used to evaluate the relative strength/frequency of El Niño and La Niña events. In contrast to previous studies, we use robust and resistant statistics to quantify the spread and symmetry of the δ18O distributions; an approach motivated by the relatively small sample size and the presence of outliers. Furthermore, we use a pseudo-proxy approach to investigate the effects of the different paleo-environmental factors on the statistics of the δ18O distributions, which could bias the paleo-ENSO reconstruction. We find no systematic difference in the magnitude/strength of ENSO during the Northern Hemisphere MCA or LIA. However, our results suggest that ENSO during the MCA was skewed toward stronger/more frequent La Niña than El Niño, an observation consistent with the medieval megadroughts documented from sites in western North America.” Khider, D., L. D. Stott, J. Emile-Geay, R. Thunell, and D. E. Hammond (2011), Paleoceanography, 26, PA3222, doi:10.1029/2011PA002139. [Full text]

Methane release from hydrates may already be occurring

Contribution of Oceanic Gas Hydrate Dissociation to the Formation of Arctic Ocean Methane Plumes – Reagan et al. (2011) “Vast quantities of methane are trapped in oceanic hydrate deposits, and there is concern that a rise in the ocean temperature will induce dissociation of these hydrate accumulations, potentially releasing large amounts of carbon into the atmosphere. Because methane is a powerful greenhouse gas, such a release could have dramatic climatic consequences. The recent discovery of active methane gas venting along the landward limit of the gas hydrate stability zone (GHSZ) on the shallow continental slope (150 m – 400 m) west of Svalbard suggests that this process may already have begun, but the source of the methane has not yet been determined. This study performs 2-D simulations of hydrate dissociation in conditions representative of the Arctic Ocean margin to assess whether such hydrates could contribute to the observed gas release. The results show that shallow, low-saturation hydrate deposits, if subjected to recently observed or future predicted temperature changes at the seafloor, can release quantities of methane at the magnitudes similar to what has been observed, and that the releases will be localized near the landward limit of the GHSZ. Both gradual and rapid warming is simulated, along with a parametric sensitivity analysis, and localized gas release is observed for most of the cases. These results resemble the recently published observations and strongly suggest that hydrate dissociation and methane release as a result of climate change may be a real phenomenon, that it could occur on decadal timescales, and that it already may be occurring.” Reagan, M. T., G. J. Moridis, S. Elliott, and M. E. Maltrud (2011), J. Geophys. Res., doi:10.1029/2011JC007189, in press.

Not much change to ENSO during 21st century

Will there be a significant change to El Niño in the 21st century? – Stevenson et al. (2011) “The El Niño/Southern Oscillation (ENSO) response to anthropogenic climate change is assessed in the following 1° nominal resolution CCSM4 CMIP5 simulations: 20th century ensemble, pre-industrial control, 21st century projections and stabilized 2100–2300 ‘extension runs’. ENSO variability weakens slightly with CO2; however, various significance tests reveal that changes are insignificant at all but the highest CO2 levels. Comparison with the 1850 control simulation suggests that ENSO changes may become significant on centennial timescales; the lack of signal in the 20th vs. 21st century ensembles is due to their limited duration. Changes to the mean state are consistent with previous studies: a weakening of the subtropical wind stress curl, an eastward shift of the tropical convective cells, a reduction in the zonal SST gradient and an increase in vertical thermal stratification take place as CO2 increases. The extratropical thermocline deepens throughout the 21st century, with the tropical thermocline changing slowly in response. The adjustment timescale is set by the relevant ocean dynamics, and the delay in its effect on ENSO variability is not diminished by increasing ensemble size. The CCSM4 results imply that 21st century simulations may simply be too short for identification of significant tropical variability response to climate change. An examination of atmospheric teleconnections, in contrast, shows that the remote influences of ENSO do respond rapidly to climate change in some regions, particularly during boreal winter. This suggests that changes to ENSO impacts may take place well before changes to oceanic tropical variability itself becomes significant.” Samantha Stevenson and Baylor Fox-Kemper, Markus Jochum, Richard Neale, Clara Deser and Gerald Meehl, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00252.1.

Warming sea surface shifts storm tracks poleward

Changes in the extra-tropical storm tracks in response to changes in SST in an AGCM – Graff & Lacasce (2011) “A poleward shift in the extra-tropical storm tracks has been indentified in observational and climate simulations. We examine the role of altered sea surface temperatures (SSTs) on the storm track position and intensity in an atmospheric general circulation model (AGCM) using realistic lower boundary conditions. A set of experiments were conducted in which the SSTs where changed by 2K in specified latitude bands. The primary profile was inspired by the observed trend in ocean temperatures, with the largest warming occurring at low-latitudes. The response to several other heating patterns was also investigated, to examine the effect of imposed gradients and low-vs. high-latitude heating. We focus on the Northern Hemisphere (NH) winter, averaged over a 20 year period. Results show that the storm tracks respond to changes in both the mean SST and SST gradients, consistent with previous studies employing aquaplanet (water-only) boundary conditions. Increasing the mean SST strengthens the Hadley circulation and the sub-tropical jets, causing the storm tracks to intensify and shift poleward. Increasing the SST gradient at mid-latitudes similarly causes an intensification and a poleward shift of the storm tracks. Increasing the gradient in the tropics on the other hand causes the Hadley cells to contract and the storm tracks to shift equatorward. Consistent shifts are seen in the mean zonal velocity, the atmospheric baroclinicity, the eddy heat and momentum fluxes, and the atmospheric meridional overturning circulation. The results support the idea that oceanic heating could be a contributing factor to the observed shift in the storm tracks.” Lise Seland Graff and J. H. Lacasce, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00174.1.

Arctic Ocean net primary production has increased

Secular trends in Arctic Ocean net primary production – Arrigo & van Dijken (2011) “A satellite-based study was conducted to document daily changes in net primary production (NPP) by phytoplankton in the Arctic Ocean from 1998 to 2009 using fields of sea ice extent, sea surface temperature, and chlorophyll a concentrations. Total annual NPP over the Arctic Ocean exhibited a statistically significant 20% increase between 1998 and 2009 (range = 441–585 Tg C yr−1), due mainly to secular increases in both the extent of open water (+27%) and the duration of the open water season (+45 days). Increases in NPP over the 12 year study period were largest in the eastern Arctic Ocean, most notably in the Kara (+70%) and Siberian (+135%) sectors. NPP per unit area for the Arctic Ocean averaged 101 g C m−2 yr−1 with no significant change over the study period. In the western sectors, NPP ranged from 71.3 ± 11.0 g C m−2 yr−1 in the Beaufort to 96.9 ± 7.4 g C m−2 yr−1 in the Chukchi, while in the more productive eastern Arctic, annual NPP between 1998 and 2009 ranged from 101 ± 15.8 in the Siberian sector to 121 ± 20.2 in the Laptev. Results of a statistical analysis suggest that between 1979 and 1998 (prior to the launch of SeaWiFS and MODIS), total Arctic NPP likely averaged 438 ± 21.5 Tg C yr−1. Moreover, when summer minimum ice cover drops to zero sometime during the first half of this century, annual NPP in the Arctic Ocean could reach ∼730 Tg C yr−1. Nutrient fluxes into Arctic surface waters need to be better understood to determine if these projected increases are sustainable.” Arrigo, K. R., and G. L. van Dijken (2011), J. Geophys. Res., 116, C09011, doi:10.1029/2011JC007151.

New ice core record shows prominent volcanic eruption in 531 A.D.

South Pole ice core record of explosive volcanic eruptions in the first and second millennia A.D. and evidence of a large eruption in the tropics around 535 A.D. – Ferris et al. (2011) “A record of explosive eruptions over the last 1830 years reconstructed from a South Pole, Antarctica, ice core extends the coverage of volcanic history to the start of the first millennium A.D. The ice core dating by annual layer counting carries an uncertainty of ±2% of the number of years from time markers, with the largest dating error of ±20 years at the bottom of the 182 m core. Several aspects of the methodology of detecting and quantifying volcanic sulfate signals in ice cores are examined in developing this record. The new record is remarkably consistent with previous South Pole records. A comparison with records from several Antarctica locations suggests that the South Pole location is among the best for ice core reconstruction of volcanic records, owing to the excellent preservation of volcanic signals at the South Pole, the relatively low and uniform sulfate background, and the moderately high snow accumulation rates which allow for dating by annual layer counting. A prominent volcanic event dated at 531(±15) A.D., along with evidence from other records, indicates that an unusually large eruption took place in the tropics and was probably responsible for the “mystery cloud” climate episode of 536–537 A.D. The date of 536 is suggested for a prominent volcanic signal that appears in the first half of the sixth century A.D. in ice cores, which can in turn be used as a time stratigraphic marker in dating ice cores by annual layer counting or by computing average accumulation rates or layer thicknesses with such markers.” Ferris, D. G., J. Cole-Dai, A. R. Reyes, and D. M. Budner (2011), J. Geophys. Res., 116, D17308, doi:10.1029/2011JD015916.

Northwards shift of shrub tundra might not lead to decrease in albedo

The response of Arctic vegetation to the summer climate: relation between shrub cover, NDVI, surface albedo and temperature – Blok et al. (2011) “Recently observed Arctic greening trends from normalized difference vegetation index (NDVI) data suggest that shrub growth is increasing in response to increasing summer temperature. An increase in shrub cover is expected to decrease summer albedo and thus positively feed back to climate warming. However, it is unknown how albedo and NDVI are affected by shrub cover and inter-annual variations in the summer climate. Here, we examine the relationship between deciduous shrub fractional cover, NDVI and albedo using field data collected at a tundra site in NE Siberia. Field data showed that NDVI increased and albedo decreased with increasing deciduous shrub cover. We then selected four Arctic tundra study areas and compiled annual growing season maximum NDVI and minimum albedo maps from MODIS satellite data (2000–10) and related these satellite products to tundra vegetation types (shrub, graminoid, barren and wetland tundra) and regional summer temperature. We observed that maximum NDVI was greatest in shrub tundra and that inter-annual variation was negatively related to summer minimum albedo but showed no consistent relationship with summer temperature. Shrub tundra showed higher albedo than wetland and barren tundra in all four study areas. These results suggest that a northwards shift of shrub tundra might not lead to a decrease in summer minimum albedo during the snow-free season when replacing wetland tundra. A fully integrative study is however needed to link results from satellite data with in situ observations across the Arctic to test the effect of increasing shrub cover on summer albedo in different tundra vegetation types.” Daan Blok et al 2011 Environ. Res. Lett. 6 035502 doi:10.1088/1748-9326/6/3/035502. [Full text]

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2 Responses to “New research from last week 37/2011”

  1. Benjamin Franz said

    ‘Contribution of Oceanic Gas Hydrate Dissociation to the Formation of Arctic Ocean Methane Plumes – Reagan et al. (2011)’ has changed URLs. The new URL is http://www.agu.org/pubs/crossref/2011/2011JC007189.shtml

  2. Ari Jokimäki said

    Fixed, thank you. :)

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